Manufacture of microporous metallic tubes consisting mainly of nickel



United States Patent 3,001,871 MANUFACTURE OF MICROPOROUS METALLIC TUBES CONSISTING MAINLY 0F NICKEL Nguyen Thien-Chi and Pierre Plurien, Paris, France, as-

signors to Commissariat a lEnergie Atomique, Paris, France, a state administration No Drawing. Filed May 1, 1958, Ser. No. 732,141 Claims priority, application France May 3, 1957 7 Claims. (Cl. 75211) The present invention relates to the manufacture of microporous metallic tubes consisting mainly of nickel or a nickel alloy.

It is known that for some works it is necessary to make use of tubes of this kind of rectilinear shape and having very thin walls (averaging A of a mm. of thickness) the pores of which have a radius averaging 0.01 micron, which tubes must have a good surface state, a high permeability to gases and be capable to resisting the chemical corrosive action of some gases, for instance uranium hexafluoride.

Now, the methods of conventional metallurgy and powder metallurgy known at the present time do not permit of obtaining microporous tubes complying with these conditions.

The chief object of our invention is to provide a method of making tubes which comply with said conditions.

The method according to our invention essentially com prises crushing in the moist state a powder of a material of the group consisting of nickel and nickel containing oxides, this crushing operation being performed in such manner as to do away with agglomerates of particles as may exist in the starting material, coating the grains formed by this crushing operation with a binder volatile at a temperature ranging from 200 to 400 C., extruding the paste thus obtained to give it the form of tubes, dehydrating and drying these tubes, and subjecting said dried tubes to a sintering treatment.

Advantageously, the starting material is a powder ob tained by a reduction of a formiate or an oxalate of nickel at a temperature ranging from 150 to 750 C. at atmospheric pressure in the presence of hydrogen.

The coating of the grains of powder is preferably performed in two steps, the first step consisting in adding the binder to said grains at the end of the crushing operation while pursuing said operation, the second step consisting in extruding the paste thus obtained through nozzles to form vermicular threads which are again agglomerated into a paste and passed repeatedly through said nozzles, some more binder being added to said vermicular threads between at least some of these extrusion operations.

The binder may be advantageously constituted by gum tragacanth or an alginate, the amount of binder ranging from 2 to of the weight of the powder to be coated, and being preferably about 7%.

The extrusion of the paste to form the tubes is performed at a speed ranging from 0.1 to 10 centimeters per second, and preferably approximating 1 cm. per second.

The dehydrating operation is effected by placing the tubes in a dehydrating bath of a composition corresponding to the nature of the binder and which is preferably made of acetone when the binder is gum tragacanth.

The sintering operation is performed at a temperature ranging from 500 to 900 C. in an atmosphere of hydrogen or an inert gas or in a vacuum.

The tubes to be sintered are placed on smooth cylindrical supports of a diameter corresponding substantially to the desired inner diameter of the finished tubes, the whole being placed in a mufile introduced vertically in the sintering furnace.

We will now describe some examples of the manufac- 3,001,871 Patented Sept. 26, 1961 ture of microporous nickel tubes according to the present invention.

These examples relate to the production of nickel tubes having a diameter of 15 mm. and a thickness of 0.2 mm. In these examples, the steps concerning the obtainment of the starting material, the preparation of the paste and the extrusion thereof are the same. The only difierence are concerned with the sintering conditions.

The qualities of these tubes will be defined by the mean radius of their pores in microns and by their permeability.

The means radius corresponds to the separation, by gaseous diffusion through the wall of one of these tubes, of a mixture containing equal parts of nitrogen and carbonic acid gas. Permeability is the rate of flow in molecule grams of air per minute through one square centimeter of tube wall with a difference of pressure of one centimeter of mercury between opposite sides of said wall.

The nickel formiate is reduced in a mufile tunnelfurnace at a temperature of 350 C., for one hour and a half, in a small stream of hydrogen. The gases produced by the reduction of said formiate are themselves reducing gases and participate in the reduction reaction.

The powder that is obtained consists of agglomerates of extremely fine grains which must be broken. For this purpose, we proceed to a moist crushing in nickel vessels containing nickel balls. In the example that is described, each of said vessels contains 5.5 kg. of powder and 3 liters of distilled water, and the crushing is pursued for 18 days. We obtain a pasty suspension to which we add, in every vessel, 44 grams of gum tragacanth in powder state and one liter of distilled water. The crushing apparatus is again caused to work for 24 hours, then 66 grams of gum tragacanth are again added and the apparatus is again caused to work for 24 hours. We thus obtain a liquid. gel containing as a whole 2% by weight of binder '(gum tragacanth) which is dried at 40 C. in a drying room where air is circulated, until a pasty mass of substantial consistency is obtained.

The first step above described of the coating of the grains with the gum tragacanth has for its object to coat every grain of metallic powder with gum tragacanth, but repeated shocks destroy the plastifying properties of the binder and it is necessary to proceed, in a second step, to an improvement of this coating. This is obtained in an extrusion apparatus provided with a multiplicity of holes or nozzles which, in the example that is described, have a diameter of 1.5 mm. Such holes or nozzles give rise to vermicular threads similar to vermicelli. To these vermicular threads we add a powder of gum tragacanth and the paste formed by these threads is passed once more through the nozzles or holes, the operation being repeated several times until we obtain vermicular threads which are not sticky. The repeated addition of gum tragacanth is thus effected until it corresponds to about 5% of the weight of nickel contained in the paste, which with the 2% previously added during the crushing operation, corresponds to a total of 7%. After this, when the vermicular threads seem quite homogeneous, the extrusion pressure is reduced and a small amount of distilled water is added to the paste. After this addition of water, the paste is again passed through the extrusion device about ten times so as to obtain a perfect homogeneity.

The nozzles previously used are then replaced by cylindrical nozzles with a central spindle. The diameter of each of said nozzles is 22 mm. and that of the spindle is 21.5 mm. The apparatus is provided with a device for centering the spindle constituted by three screws at C. placed under the spindle-carrier. The paste is compressed in the extrusion apparatus at a pressure ranging from 10 to 15 tons and it flows in the form of tubes at the outlet of each nozzle at the rate of about 1 cm. per second. The centering screws are adjusted so as to obtain a straight tube which is received in a tank containing acetone, in order to harden the tube obtained by extrusion, this acetone having a dehydrating action. The tube that is obtained, of a length of about 1 meter, is removed from the tank after some minutes and placed in a glass tube filled with acetone, the inner diameter of this tube being some millimeters greater than the outer diameter of the extruded tube.

After one hour, each glass tube is emptied from the acetone present therein and closed by means of porous plugs in order to avoid too quick a drying of the extruded tube which would tend to break down. The tubes are then placed in a cold drying room for eight days. For one subsequent week, the drying room is then heated at 30 C. After cooling, the tubes are ready to be sintered.

The dry tube is extracted from the glass tube and placed on a metallic cylindrical support of nickel or cupro-nickel for instance, covered with alumina in order to prevent sticking during the sintering operation. The diameter of the cylindrical support must be very slightly smaller than the inner diameter of the sintered tube to be obtained. During the sintering operation, the tube is applied without tightening upon the support. Thus we obtain tubes which are quite straight.

The support is placed with its tube in a cylindrical metallic mufile of an inner diameter equal to about 30 mm. The mufile is introduced vertically in an electric furnace at room temperature.

Example I In this example, the sintering operation is conducted in a vacuum in the following conditions: the end of the muflie that is projecting from the furnace is connected with a rotary pump. When the vacuum reaches mm. of mercury, the furnace is slowly heated to 700 C. This temperature is maintained for one hour and the muffle is taken from the furnace and cooled in air.

The tube that is obtained has the following characteristics: mean radius of the pores, 0.03 micron; permeability, 340.10-

Example I] In this example, the sintering operation is conducted in a vacuum in the following conditions: the end of the muflle that is projecting from the furnace is connected with a rotary pump. When the vacuum reaches 10- mm. of mercury, the furnace is gradually heated to 350 C., at which temperature a very important degasing takes place. The furnace is held at this temperature until a vacuum of at least 10- mm. of mercury is obtained, after which the temperature is slowly brought to the sintering temperature which is about 700 C. After one hour at this temperature, the muflie is withdrawn from the furnace and cooled in air.

The tube that is obtained has the following characteristics: mean radius of the pores, 0.01 micron, permeability, 20010- Example III In this example, the sintering operation is conducted in hydrogen in the following conditions: at room temperature, a stream of hydrogen is caused to pass through the mufflc at the rate of about 600 liters per hour. When the muflie is purified, the hydrogen stream is ignited at the outlet thereof and the temperature of the furnace is quickly brought to the sintering temperature which ranges from 500 to 900 C. After half an hour at this temperature, the tube is withdrawn and cooled in hydrogen or in a neutral gas.

(For a tube sintered at 650 C., the characteristics are as follows: mean radius of the pores, 0.04 micron; permeability, 400.10-

For a tube sintered at 600 C., the characteristics are as follows: mean radius of the pores, 0.025 micron; permeability: 280.10'.

Example IV In this example, the sintering operation in conducted by combining the two processes of Examples III and I, that is to say a sintering in hydrogen followed by a sintering in a vacuum (the sintering temperature in a vacuum being from to 200 C. higher than that of the sintering operation in hydrogen).

The characteristics of the tube that is obtained are as follows: mean radius of the pores, 0.03 micron; permeability, 30010- What we claim is:

1. The method of making a thin-walled microporous tube which comprises starting from a powder obtained by reduction of a nickel salt of the group consisting of nickel formiate and nickel oxalate at a temperature ranging from to 750 C. at atmospheric pressure in a reducing atmosphere, adding water to the powder thus obtained to form a paste and crushing paste until the nickel particle agglomerates have been destroyed, leaving only grains the means diameter of which is of an order of magnitude not higher than 0.1 micron, mixing with this paste a binder of the group consisting of gum tragacanth and alginates so that said grains are coated individually with said binder, extruding the paste thus obtained through fine nozzles to form vermicular threads, said threads being reagglomerated into a paste and this last mentioned paste being again passed through said nozzles, this last operation being repeated a plurality of times, adding to said vermicular threads, between at least some of these repeated extrusion operations, a binder of the group consisting of gum tragacanth and alginates so as to form a paste, extruding the last mentioned paste to give it the form of a tube, this last mentioned extrusion being effected under a pressure ranging from 600 to 1000 kgs. per sq. mm. of extrusion area, dehydrating and drying said tube, placing said tube on a smooth cylindrical support of a diameter corresponding to the desired inner diameter of the finished tube, and subjecting the dried tube to a sintering treatment, performed at a temperature ranging from 500 to 900 C. for a time ranging from 30 to 90 minutes.

2. A method according to claim 1 in which the amount of binder ranges from 2 to 10% of the weight of powder.

3. A method according to claim 1 in which the extrusion of the paste to form the tubes is performed at a speed ranging from 0.1 to 10 centimeters per second.

4. A method according to claim 1 in which the dehydrating operation is effected by placing the tubes in a dehydrating bath of a composition corresponding to the nature of the binder.

5. A method according to claim 1 in which the sintering operation is performed in a hydrogen atmosphere.

6. A method according to claim 1 in which the sintering operation is performed in a vacuum.

7. A method according to claim 1 in which the sintering operation is performed in an inert gas atmosphere.

References Cited in the file of this patent UNITED STATES PATENTS 2,792,302 Mott May 14, 1957 2,794,735 Schlecht June 4, 1957 2,834,673 Lynch et al May 13, 1958 2,857,270 Brundin Oct. 21, 1958 

1. THE METHOD OF MAKING A THIN-WALLED MICROPOROUS TUBE WHICH COMPRISES STARTING FROM A POWDER OBTAINED BY REDUCTION OF A NICKEL SALT OF THE GROUP CONSISTING OF NICKEL FORMIATE AND NICKEL OXALATE AT A TEMPERATURE RANGING FROM 150 TO 750*C. AT ATMOSPHERIC PRESSURE IN A REDUCING ATMOSPHERE, ADDING WATER TO THE POWDER THUS OBTAINED TO FORM A PASTE AND CRUSHING PASTE UNTIL THE NICKEL PARTICLE AGGLOMERATES HAVE BEEN DESTROYED, LEAVING ONLY GRAINS THE MEANS DIAMETER OF WHICH IS OF AN ORDER OF MAGNITUDE NOT HIGHER THAN 0.1 MICRON, MIXING WITH THIS PASTE A BINDER OF THE GROUP CONSISTING OF GUM TRAGACANTH AND ALGINATES SO THAT SAID GRAINS ARE COATED INDIVIDUALLY WITH SAID BINDER, EXTRUDING THE PASTE THUS OBTAINED THROUGH FINE NOZZLES TO FORM VERMICULAR THREADS, SAID THREADS BEING REAGGLOMERATED INTO A PASTE AND THIS LAST MENTIONED PASTE BEING AGAIN PASSED THROUGH SAID NOZZLES, THIS LAST OPERATION BEING REPEATED A PLURALITY OF TIMES, ADDING TO SAID VERMICULAR THREADS, BETWEEN AT LEAST SOME OF THESE REPEATED EXTRUSION OPERATIONS, A BINDER OF THE GROUP CONSISTING OF GUM TRAGACANTH AND ALGINATES SO AS TO FORM A PASTE, EXTRUDING THE LAST MENTIONED PASTE TO GIVE IT THE FORM OF A TUBE, THIS LAST MENTIONED EXTRUSION BEING EFFECTED UNDER A PRESSURE RANGING FROM 600 TO 1000 KGS. PER SQ. MM. OF EXTRUSION AREA, DEHYDRATING AND DRYING SAID TUBE, PLACING SAID TUBE ON A SMOOTH CYLINDRICAL SUPPORT OF A DIAMETER CORRESPONDING TO THE DESIRED INNER DIAMETER OF THE FINISHED TUBE, AND SUBJECTING THE DRIED TUBE TO A SINTERING TREATMENT, PERFORMED AT A TEMPERATURE RANGING FROM 500 TO 900* C. FOR A TIME RANGING FROM 30 TO 90 MINUTES. 